The present invention relates to a ball forming device used in a bonding apparatus and to a ball forming method.
Gold wire is commonly used as a bonding wire (merely called, “wire”) for connecting between a first bonding point and a second bonding point and for forming bumps. However, in order to reduce manufacturing costs, attempts have been made to use copper wires or aluminum wires. However, these wires cause oxidation to readily occur when forming a ball at a tip end of the wire. So as to prevent oxidations, various structures have been proposed, and Japanese Patent Application Examined Publication Disclosure No. H04 (1992)-28136 B, which is corresponding to Japanese Patent Application Unexamined Publication Disclosure S60 (1985)-244034 A, for example, discloses a ball forming device that prevents wire oxidation in a wire bonding apparatus.
In the ball forming device of Japanese Patent Application Examined Publication Disclosure No. H04 (1992)-28136 B, a cylindrical cover is provided so that it surrounds the capillary, and a reducing gas or inactive gas is supplied inside this cylindrical cover to bring the vicinity of a tip end of the wire passing through the capillary into a gas atmosphere. A ball is formed at the tip end of the wire by an electric discharge from an electronic flame off prove in this gas atmosphere.
In this ball forming device of Japanese Patent Application Examined Publication Disclosure No. H04 (1992)-28136, though not shown in its drawings, the upper part of the capillary is secured to a bonding arm, and the work to be bonded is pressed against a working platform by a pressing arm that moves up and down, vertically. For this reason, in view of the length (or height) of the capillary, the cylindrical cover needs to have a height that does not interfere with the pressing arm that moves up and down and the thickness of the bonding arm. Generally, the length of the capillary is 11.1 mm, the length by which the capillary is secured to the bonding arm is 4 mm, and the thickness and the distance of the up-and-down motion of the pressing arm is 4.7 mm; accordingly, the height of the cylindrical cover is as thin as approximately 2 mm or so. With such a very thin cylindrical cover, it is very difficult to form a gas atmosphere inside the cylindrical cover. Furthermore, the capillary moves up and down at high speed inside the cylindrical cover, and it also moves in the directions of X and Y axes (or horizontally); accordingly, with such a very thin cylindrical cover, the gas can easily escape. Therefore, in order to make the cylindrical cover long and form a sufficient gas atmosphere inside this cylindrical cover, it is necessary to make the length of the capillary substantially long; however this is difficult to accomplish because when the capillary has a substantial length, then the resonance point of the capillary brought by an ultrasonic wave oscillator tends to change so that smooth ultrasonic transmission is hindered.